This book isn't really as bad as the other review makes out. Yes, the map is screwed up, but the table of contents is fine, and sorted by area. Yes, many routes include on road sections, but that is the nature of the area, and doesn't particularly bother me. The book is somewhat out of date: for example there is no Marin trail for Gwydyr Forest, and no routes for Coed-y-Brenin (both are briefly mentioned in an addendum at back of the book). The substantial introduction makes little or no mention of mountain-biking in general, or more specifically in the area, and is mostly taken up with how to read a map.
However, there aren't many MTB books specifically for this area. The only other books I know of (Bikefax Snowdonia and Bikefax North East Wales), which I also own, are either out of print and difficult to obtain for a reasonable price (Snowdonia), still out of date (though not as badly as this book, 2005 versus 1995 -- e.g. part of the route through Dinorwic quarry is now officially closed off, though it appears unofficially open, judging by the graffiti on no entry signs!), and which have their own problems anyway.
This book makes a good companion to the Bikefax books, given the lack of alternatives, as there isn't a great deal of overlap between this and the Bikefax books. Download free official maps for the Marin trail, Coed-y-Brenin, etc., online, or you can pick them up cheaply in tourist information centers in the area (there's a pack containing a whole set of routes for the area).